Alternate Accounts (often referred to as Alts) are Minecraft accounts used by a player other than their main account. This is often due to a multitude of reasons, including raiding, vault defense, vault attacks, espionage, botting, travel, and possibly creating a fresh start for disgraced players. Different servers have had different rules regarding the number of accounts a person can use at any given time, and there have often been limits placed in order to limit the advantages they could have regardless of the number allowed on the server.
Known Server Restrictions/Limits on Alts
The two most common restrictions are alt-banning the accounts of pearled players, and limiting the number of alts a player can have. Both restrictions occur by barring any further accounts from the same IP from entering the server. VPNs are often also banned in tandem to prevent players from getting around this limit. However, this has posed a problem in certain situations before, such as two distinct players using the same IP to play on the server (Most likely due to them knowing each other in real life). Accounts that are from the same IP are often "linked" so that the server knows what associated accounts to ban or bar from entering, however it can't tell when the linked accounts could be the accounts of two different people, and may require admin intervention if it causes problems for either players.